Think about better DNS server integration

[fxkr]

Generating zonefiles may be robust, but it seems rather hacky. It would be nice if there was zero custom software on the DNS server.

Ideas:

• Write own DNS server that acts as hidden master, supports DNS AXFR + NOTIFY, and uses zonefile data in backend. When to do DNS NOTIFY can be found out via PostgreSQL TRIGGER + NOTIFY + LISTEN.
• nsudpate. Main problem: only works for single records, not entire zone. May lead to stale records if deletion event is gets lost.
• Use PowerDNS, replicate netbox DB records directly into PowerDNS DB records. Disadvantage: specific to PowerDNS.

Shared problem of "push" based approaches: we need to traverse our NAT...